A dynamic gain equalization filter (DGEF) is a device or arrangement that is useful for controlling wavelength-division multiplexed (WDM) channel powers, especially in ultra-long-haul (>100 km) optical communication systems. Presently, there are fiber-based solutions, for example, as described in H. S. Kim, S. H. Yun, H. K. Kim, N. Park, and B. Y. Kim, “Actively gain-flattened erbium-doped fiber amplifier over 35 nm by using all-fiber acousto-optic tunable filters,” IEEE Photon. Technol. Lett., Vol. 10, pp. 790-792, June 1998, and bulk-optic-based solutions, as described for example in J. E. Ford and J. A. Walker, “Dynamic spectral power equalization using micro-opto-mechanics,” IEEE Photon. Technol. Lett., Vol. 10, pp. 1440-1442, October 1998, and in T. Huang, J. Huang, Y. Liu, M. Xu, Y. Yang, M. Li, C. Mao, and J.-C. Chiao, “Performance of a liquid-crystal optical harmonic equalizer,” Optical Fiber Communication Conference, PD29 Jan. 3, 2001.
An integrated waveguide DGEF has also been considered, because of its good spectral resolution, (see, for example, C. R. Doerr, L. W. Stulz, R. Pafchek, L. Gomez, M. Cappuzzo, A. Paunescu, E. Laskowski, L. Buhl, H. K. Kim, and S. Chandrasekhar, “An automatic 40-wavelength channelized equalizer,” IEEE Photon. Technol. Lett., Vol. 12, pp. 1195-1197, 2000) as well as its solid-state reliability and large-volume manufacturability, as described in K. Inoue, T. Kominato, and H. Toba, “Tunable gain equalization using a Mach-Zehnder optical filter in multistage fiber amplifiers,” IEEE Photon. Technol. Lett., Vol. 3, pp. 718-720, 1991, as well as in A. Ranalli and B. Fondeur, “Planar tapped delay line based, actively reconfigurable gain-flattening filter,” European Conference on Optical Communication, 2000 and in B. J. Offrein, F. Horst, G. L. Bona, R. Germann, H. W. M. Salemink, and R. Beyeler, “Adaptive gain equalizer in high-index-constrast SiON technology,” IEEE Photon. Technol. Lett., Vol. 12, pp. 504-506, 2000.
A waveguide DGEF can also have a low insertion loss; previously, by putting the lossy wavelength-selective part inside one arm of a large Mach-Zehnder interferometer, it was shown by C. R. Doerr, C. H. Joyner, and L. W. Stulz, “Integrated WDM dynamic power equalizer with potentially low insertion loss,” IEEE Photon. Technol. Lett., Vol. 10, pp. 1443-1445, October 1998, that a die loss <2-dB could be achieved. However, because the known design had long dissimilar interfering paths, it required polarization diversity to achieve low polarization-dependent loss (PDL), undesirably making the final device loss at least 4.5 dB and costly. See C. R. Doerr, K. W. Chang, L. W. Stulz, R. Pafchek, Q. Guo, L. Buhl, L. Gomez, M. Cappuzzo, and G. Bogert, “Arrayed waveguide dynamic gain equalization filter with reduced insertion loss and increased dynamic range,” IEEE Photon. Technol. Lett., Vol. 13, pp. 329-331, April 2001. Also, a design using a large interferometer is undesirably sensitive to the environment.